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Evidence of Abrupt Climate Change

Researchers first became intrigued by abrupt climate change when they discovered striking evidence of large, abrupt, and widespread changes preserved in paleoclimatic archives. Interpretation of such proxy records of climate—for example, using tree rings to judge occurrence of droughts or gas bubbles in ice cores to study the atmosphere at the time the bubbles were trapped—is a well-established science that has grown much in recent years. This chapter summarizes techniques for studying paleoclimate and highlights research results. The chapter concludes with examples of modern climate change and techniques for observing it. Modern climate records include abrupt changes that are smaller and briefer than in paleoclimate records but show that abrupt climate change is not restricted to the distant past.

INTERPRETATION OF PAST CLIMATIC CONDITIONS FROM PROXY RECORDS

Paleoclimatic interpretation relies ultimately on the use of the present or recent instrumental records as the key to the past. To accomplish this, modern values observed for a given characteristic of the climate system are compared with some record from the past, such as tree-ring thickness or the isotopic composition of water frozen in ice cores (see Plates 1 and 2). Detailed understanding of these records—how the thickness of tree rings



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Abrupt Climate Change: Inevitable Surprises 2 Evidence of Abrupt Climate Change Researchers first became intrigued by abrupt climate change when they discovered striking evidence of large, abrupt, and widespread changes preserved in paleoclimatic archives. Interpretation of such proxy records of climate—for example, using tree rings to judge occurrence of droughts or gas bubbles in ice cores to study the atmosphere at the time the bubbles were trapped—is a well-established science that has grown much in recent years. This chapter summarizes techniques for studying paleoclimate and highlights research results. The chapter concludes with examples of modern climate change and techniques for observing it. Modern climate records include abrupt changes that are smaller and briefer than in paleoclimate records but show that abrupt climate change is not restricted to the distant past. INTERPRETATION OF PAST CLIMATIC CONDITIONS FROM PROXY RECORDS Paleoclimatic interpretation relies ultimately on the use of the present or recent instrumental records as the key to the past. To accomplish this, modern values observed for a given characteristic of the climate system are compared with some record from the past, such as tree-ring thickness or the isotopic composition of water frozen in ice cores (see Plates 1 and 2). Detailed understanding of these records—how the thickness of tree rings

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Abrupt Climate Change: Inevitable Surprises changes in recent wet and dry periods—lets scientists draw inferences about the past, and these records come to be considered “proxies,” or indicators of the past environment. The assumption of constancy of the relation between climate and its proxy might require little more to support it than constancy of physical law (for example, the assumption that in the past heat flowed from warm to cold rocks in the same way as today). Other assumptions might involve greater uncertainty (for example, the assumption that under different climatic conditions, marine organisms grew most vigorously during the same season and at the same water depth as in the modern environment). Testing of the underlying assumption that the present is the key to the past relies largely on the consistency of results from a wide array of proxies, particularly those depending on few assumptions. The use of multiple indicators increases the reliability of many paleoclimate reconstructions. The following pages provide a brief synopsis of paleoclimate proxies (Table 2.1) and age indicators. The description is not exhaustive and is intended only to orient the reader to some of the current paleoclimatic tools available. For more detailed reviews of methods involved in paleoclimatic interpretation see Broecker (1995), Bradley (1999), or Cronin (1999). Physical paleoclimatic indicators often rely on the fewest assumptions and so can be interpreted most directly. For example, old air extracted from bubbles in ice cores and old water from pore spaces in seabed sediments or continental rocks provide direct indications of past compositions of atmosphere, oceans, and groundwater (see Plate 1). Anomalously cold buried rocks or ice have not finished warming from the ice age and thus provide evidence that conditions were colder in the past. Conditions are also judged from the concentrations of noble gases found dissolved in old groundwaters. Some such records are subject to substantial loss of information through diffusion of the components being analyzed, which limits the ability to interpret older events. Physical indicators include the characteristics of sediments and land features. For example, the presence of sand dunes can indicate past arid conditions, and glacially polished bedrock is an indication of prior glacial conditions. Isotopic indicators are widely used in paleoclimate science. The subtle differences in behavior between chemically similar atoms having different weights (isotopes) prove to be sensitive indicators of paleoenvironmental conditions. One common application is paleothermometry. The physical and chemical discrimination of atoms of differing isotopic mass increases with decreasing temperature. For example, carbonate shells grow-

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Abrupt Climate Change: Inevitable Surprises TABLE 2.1 Paeloclimatic Proxies Paleoclimatic Recorder Climate Variable Recorded Property Measured Ice Atmospheric composition Trapped bubbles   Windiness Dust grain sizes   Source strength of wind-blown materials Abundance of pollen, dust, sea salt   Temperature Ice isotopic ratios Borehole temperatures Gas isotopes Melt layers   Snow accumulation rate Thickness of annual layers In-situ radiocarbon Ocean sediments and corals Temperature Species assemblages Shell geochemistry Alkenone (U37K' ) thermometry   Salinity Shell isotopes after correction for temperature and ice volume   Ice volume Isotopic composition of pore waters Shell isotopes after correction for temperature and salinity   pH Ocean circulation Boron isotopes in shells Cd/Ca in shells Carbon-isotopic data   Corrosiveness/chemistry of ambient waters Shell dissolution Lake and bog sediments Temperature Species assemblages Shell geochemistry   Atmospheric temperature and soil moisture Washed- or blown-in materials including pollen and spores Macrofossils such as leaves, needles, beetles, midge flies, etc.   Water balance (precipitation minus evaporation Species assemblages Shell geochemistry Tree rings Temperature and/or moisture availability Ring width or density of trees stressed by cold or drought   Variations in the isotopic ratio of water related to temperature Cellulose isotopic ratios Speleothems/cave formations Moisture availability Growth rate of formations Isotopic ratios of water related to temperature or precipitation rate Oxygen isotopic composition   Overlying vegetation Carbon-isotopic composition

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Abrupt Climate Change: Inevitable Surprises Paleoclimatic Recorder Climate Variable Recorded Property Measured Terrestrial sediment types/ nature of erosion Temperature Glaciers Permafrost Snowfall/rainfall Lakes Sand dunes Glaciers Loess Windiness Loess Sand dunes Soil formation rate/moisture availability Soil profiles Loess Boreholes Temperature Direct measurements Old groundwater Temperature Isotopic and noble gas composition of water Desert varnish Moisture availability Growth rate Chemistry NOTE: Past climate conditions can be measured only through “proxies,” characteristics that give insights about past conditions. For example, gas bubbles trapped in ice can be analyzed to understand the atmosphere at the time the bubbles were trapped. This table lists examples of paleoclimatic proxies, what the proxy measures, and from where the proxy data originated. ing in water typically favor isotopically heavy oxygen and become isotopically heavier at lower temperatures. Isotopic ratios also are used to estimate the concentration of a chemical. When a chemical is common in the environment, a “favored” isotope will be used; shortage of a chemical leads to greater use of a less favored isotope. Marine photosynthesis increasingly favors the light isotope of carbon as carbon dioxide becomes more abundant, and this allows estimation of changes in carbon dioxide concentration from the isotopic composition of organic matter in oceanic sediments. Similarly, the growth of ice sheets removes isotopically light water (ordinary water) from the ocean, increasing the use of isotopically heavy oxygen from water in carbonate shells, which then provide information on the size of ice sheets over time. Stable isotopic values in organic matter also provide important information on photosynthetic pathways and so can afford insight into the photosynthesizing organisms that were dominant at a given location in the past.

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Abrupt Climate Change: Inevitable Surprises Many chemical proxies of environmental change act like isotopic ratios in the measurement of availability of a species. For example, if decreased rainfall increases the concentration of magnesium or strontium ions in lake water, they will become more common in calcium-carbonate shells that grow in that water. However, warming can also allow increased incorporation of substitute ions in shells. Such nonuniqueness can usually be resolved through use of multiple indicators. Other chemical indicators are allied to biological processes. For example, some species of marine diatoms incorporate stiffer molecules in their cell walls to offset the softening effects of higher temperature, and these molecules are resistant to changes after the diatoms die. The fraction of stiffer molecules in sediments yields an estimate of past temperatures. This analytic technique, known as alkenone paleothermometry, is increasingly used to learn about paleotemperatures in the marine environment. Biological indicators of environmental conditions typically involve the presence or absence of indicator species or assemblages of species. For example, the existence of an old rooted tree stump shows that the climate was warm and wet enough for trees, and the type of wood indicates how warm and wet the climate was; if that tree stump is in a region where trees do not grow today, the climate change is clear. In ocean and lake sediments, the microfossil species present can indicate the temperature, salinity, and nutrient concentration of the water column when they were deposited. Pollen and macrofossils preserved in sediments are important records of variability in the terrestrial environment (see Plate 3). The presence of specific organic compounds called biomarkers in sediments can reveal what species were present, how abundant they were, and other information. The complicated nature of paleoclimatic interpretation can be seen when proxies are viewed in a practical example. During ice ages, the oceans were colder, but the water in them was also isotopically heavier because light water was removed and used in growing ice sheets. Shells that grew in water during ice age intervals contain heavier isotopes owing to cooling and changes in the isotopic composition of ocean waters. The change in ocean isotopic composition can be estimated independently from the composition of pore waters in sediments, whereas the change in temperature can be estimated from both the abundance of cold- or warm-loving shells in sediment and the abundance of stiff diatom cell-wall molecules in sediments. Concentrations of non-carbonate ions substituted into calcium carbonate shells provide further information. Because there is redundancy in the available data, reliable results can be obtained.

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Abrupt Climate Change: Inevitable Surprises Any paleoclimatic record requires age estimates, and many techniques are used to obtain them. Annual layers in trees, in sediments of some lakes and shallow marine basins, in corals, and in some ice cores allow high-resolution dating for tens of thousands of years, or longer in exceptional cases. Various radiometric techniques are also used. Dates for the last 50,000 years are most commonly obtained by using radiocarbon (14C). Changes in production of radiocarbon by cosmic rays have occurred over time, but their effects are now calibrated by using annual-layer counts or other radiometric techniques, such as the use of radioactive intermediates generated during the decay of uranium and thorium and also through the potassium-argon system. Other techniques rely on measurement of accumulated damage to mineral grains, rocks, or chemicals; this permits dating on the basis of cosmogenic exposure ages, thermoluminescence, obsidian hydration, fission tracks, amino-acid racemization, and so on. Numerous techniques allow correlation of samples and assignment of ages from well-dated to initially less well-dated records. Such techniques include the identification of chemically “fingerprinted” fallout from particular volcanic eruptions, of changes in the composition of atmospheric gases trapped in ice cores, and of changes in cosmogenic isotope production or rock magnetization linked to changes in the earth’s magnetic field. THE YOUNGER DRYAS AS AN EXAMPLE OF ABRUPT CLIMATE CHANGE Sedimentary records reveal numerous large, widespread abrupt climate changes over the last 100,000 years and beyond. The best known of them is the Younger Dryas cold interval. The Younger Dryas was a nearly global event that began about 12,800 years ago when there was an interruption in the gradual warming trend that followed the last ice age. The Younger Dryas event ended abruptly about 11,600 years ago (Figures 2.1 and 2.2). Because the Younger Dryas can be tracked quite clearly in geologic records and has received extensive study, a rather detailed summary of the evidence is given here, followed by briefer reviews of other abrupt climate changes. We then target Holocene1 abrupt climate events as examples of substantial changes that have taken place when physical conditions on the earth were more similar to today. Understanding the causes of both types of abrupt 1   The Holocene is the most recent 11,000 years since the last major glacial epoch or “ice age.”

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Abrupt Climate Change: Inevitable Surprises climate change is essential for assessing the importance of their role in our climate future. Ice Core Evidence of the Younger Dryas The Younger Dryas cold reversal is especially prominent in ice-core records from Greenland, but it is also observed in ice cores from other locations. The ice-core records provide a unique perspective that demonstrates the synchronous nature of the large, widespread changes observed. Annual-layer counting in Greenland ice cores allows determination of the age, duration, and rapidity of change of the Younger Dryas event with dating errors of about one percent (Alley et al., 1993; Meese et al., 1997). Annual-layer thicknesses corrected for the effects of ice flow give the history of snow accumulation rate in Greenland (Alley et al., 1993). Concentrations of wind-blown materials—such as dust (which in central Greenland has characteristics showing its origin in central Asia [Biscaye et al., 1997]) and sea salt—reveal changes in atmospheric concentrations of these particles (Mayewski et al., 1997) after correction for variations in dilution caused by changing snow accumulation rate (Alley et al., 1995a). Gases trapped in bubbles reveal past atmospheric composition. Methane is of special interest because it probably records the global area of wetlands. Furthermore, differences between methane concentrations observed in Greenland ice cores and those from Antarctica allow inference of changes in the wetland areas in the tropics and high latitudes (Chappellaz et al., 1997; Brook et al., 1999). The combination of the isotopic record of water making up the Greenland ice (see Plate 2; Figure 1.2) (Johnsen et al., 1997; Grootes and Stuiver, 1997) and the physical temperature of the ice (Cuffey et al., 1994, 1995; Johnsen et al., 1995) yields estimates of past temperatures in central Greenland, which can be checked by using two additional thermometers based on the thermal fractionation of gas isotopes after abrupt temperature changes (Severinghaus et al., 1998). Ice-core records from Greenland thus provide high-resolution reconstructions of local environmental conditions in Greenland (temperature and snow accumulation rate), conditions well beyond Greenland (wind-blown materials including sea salt and Asian dust), and even some global conditions (wetland area inferred from methane), all on a common time scale (Figures 2.1, 2.2, and 2.3). A review of available Greenland ice-core data is given by Alley (2000). The data were collected by two international teams of investigators from multiple laboratories. The duplication shows the high reliability of the

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Abrupt Climate Change: Inevitable Surprises FIGURE 2.1 The Younger Dryas (YD) climate event, as recorded in an ice core from central Greenland and a sediment core from offshore Venezuela. The upper-most curve is the gray-scale (light or dark appearance) of the Cariaco Basin core, and probably records changes in windiness and rainfall (Hughen et al., 1998). The other curves are from the GISP2, Greenland ice core. The rate of snow accumulation and the temperature in central Greenland were calculated by Cuffey and Clow (1997), using the layer-thickness data from Alley et al. (1993) and the ice-isotopic ratios from Grootes and Stuiver (1997), respectively. The independent Severinghaus et al. (1998) temperature estimate is shown by the circle near the end of the Younger Dryas. Methane data are from Brook et al. (1996) (squares) and Severinghaus et al. (1998) (x), and probably record changes in global wetland area. Changes in the d15N values as measured by Severinghaus et al. (1998) record the temperature difference between the surface of the Greenland ice sheet and the depth at which bubbles were trapped; abrupt warmings caused the short-lived spikes in this value near the end of the Younger Dryas and near 14.7 thousand years. Highs in sea-salt sodium indicate windy conditions from beyond Greenland, and even larger changes in calcium from continental dust indicate windy and dry or low-vegetation conditions in the Asian source regions (Mayewski et al., 1997; Biscaye et al., 1997). Calcium and sodium concentrations measured in the ice have been converted to concentrations in the air over Greenland, and are displayed by dividing by the estimated average atmospheric concentrations over Greenland in the millennium before the Little Ice Age, following Alley et al. (1997). Most of the ice-core data, and many related data sets, are available on The Greenland Summit Ice Cores CD-ROM, 1997, National Snow and Ice Data Center, University of Colorado at Boulder, and the World Data Center-A for Paleoclimatology, National Geophysical Data Center, Boulder, Colorado, www.ngdc.noaa.gov/paleo/icecore/greenland/summit/index.html. Figure is modified from Alley (2000).

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Abrupt Climate Change: Inevitable Surprises data from the cores over the most recent 110,000 years, and the multiparameter analyses give an exceptionally clear view of the climate system. Briefly, the data indicate that cooling into the Younger Dryas occurred in a few prominent decade(s)-long steps, whereas warming at the end of it occurred primarily in one especially large step (Figure 1.2) of about 8°C in about 10 years and was accompanied by a doubling of snow accumulation in 3 years; most of the accumulation-rate change occurred in 1 year. (This matches well the change in wind-driven upwelling in the Cariaco Basin, offshore Venezuela, which occurred in 10 years or less [Hughen et al., 1996].) Ice core evidence also shows that wind-blown materials were more abundant in the atmosphere over Greenland by a factor of 3 (sea-salt, submicrometer dust) to 7 (dust measuring several micrometers) in the Younger Dryas atmosphere than after the event (Alley et al., 1995b; Mayewski et al., 1997) (Figure 2.1). Taylor et al. (1997) found that most of the change in most indicators occurred in one step over about 5 years at the end of the Younger Dryas, although additional steps of similar length but much smaller magnitude preceded and followed the main step, spanning a total of about 50 years. Variability in at least some indicators was enhanced near this and other transitions in the ice cores (Taylor et al., 1993), complicating identification of when transitions occurred and emphasizing the need for improved statistical and analytical tools in dealing with abrupt climate change. Beginning immediately after the main warming in Greenland (by less than or equal to 30 years), methane rose by 50 percent over about a century; this increase included tropical and high-latitude sources (Chappellaz et al., 1997; Severinghaus et al., 1998; Brook et al., 1999).

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Abrupt Climate Change: Inevitable Surprises FIGURE 2.2 The accumulation rate of ice in Greenland was low during the Younger Dryas, and both the start and end of the period show as abrupt changes. Modified from Alley et al. (1993). Ice cores from other sites, including Baffin Island, Canada (Fisher et al., 1995), Huascaran, Peru (Thompson et al., 1995), and Sajama, Bolivia (Thompson et al., 1998), show evidence of a late-glacial reversal that is probably the Younger Dryas, although the age control for these cores is not as accurate as for cores from the large ice sheets. The Byrd Station, Antarctica, ice core and possibly other southern cores (Bender et al., 1994; Blunier and Brook, 2001) indicate a broadly antiphased behavior between the high southern latitudes and much of the rest of the world, with southern warmth during the Younger Dryas interval (see Plate 2). The record from Taylor Dome, Antarctica, a near-coastal site, appears to show a slight cooling during the Younger Dryas, although details of the synchronization with other ice cores remain under discussion (Steig et al., 1998). The Southern Hemisphere records are not comparable with those from central Greenland in time resolution; further coring is planned. The ice-core records demonstrate that much of the earth was affected simultaneously by the Younger Dryas, typically with cold, dry, windy con-

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Abrupt Climate Change: Inevitable Surprises FIGURE 2.3 Climate data from the GISP2 core, central Greenland, showing changes about 8,200 years ago probably caused by outburst flooding from around the melting ice sheet in Hudson Bay (Barber et al., 1999) and affecting widespread regions of the globe. The event punctuated generally warm conditions not too different from recently, so warmth is not a guarantee of climate stability. Accumulation and temperature reflect conditions in Greenland, chloride is wind-blown sea-salt from beyond Greenland, and calcium is continental dust probably from Asia (Biscaye et al., 1997). Forest-fire smoke likely is from North America, and methane probably records global wetland area. Data are shown as approximately 50-year running means. Accumulation from Alley et al. (1993) and Spinelli (1996), chloride and calcium from O’Brien et al. (1995), and fire data shown as a 50-year histogram of frequency of fallout from fires (Taylor et al., 1996), expressed as ratios to their average values during the approximately 2,000 years just prior to the Little Ice Age. Temperature is calculated as a deviation from the average over the same 2,000 years, from oxygen-isotopic data of ice (Stuiver et al., 1995), assuming a calibration of 0.33 per mil per degree C (Cuffey et al., 1995). Methane concentrations from the GISP2 core (heavier line; Brook et al., 1996) and the GRIP core (Blunier et al., 1995) are shown in parts per billion by volume (ppb). Note that some scales increase upward and others downward, as indicated, so that all curves vary together at the major events. Modified from Alley et al. (1997).

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Abrupt Climate Change: Inevitable Surprises temperature is typically 4°C in the upper mixed layer, 1°C at 150m, and 0.1°C or less at 1000 m. Sampling must be frequent enough so as not to be aliased by this cycle. Monitoring deep, “stable” water masses is good strategy. Combining weather ship data, hydrographic work, particularly that of the CSS Hudson from Bedford Institute of Oceanography, Canada, and more recent moored instrumentation reveals a picture of climate change over the last 30 years that stands out in the 100-year record. There has been a persistent decline in salinity from top to bottom of the subpolar Atlantic Ocean and widespread cooling since 1972 (Figure 2.10) (e.g., Lazier, 1995). Accompanied by a surge of strong, cold winters with winds from the Canadian Arctic (and strongly positive NAO index), convection in the Labrador Sea has penetrated to great depth, yet episodically, during this period. The intense period of Labrador Sea convection produced a great outflow of the water mass, which has been traced along the western boundary of the Atlantic as far as the Antilles (Molinari et al., 1998; Smethie et al., 2000). The positive values of the AO/NAO-index time series in this period stand out in the 150-year record. Then, suddenly, the cold winters in the northwest Atlantic reverted to an extremely mild state in early 1996. The AO/NAO indices had suddenly reversed, taking on minimums as extreme as anything seen during the century. The ocean has responded quickly, with warm, saline water invading the Labrador Sea to replace the cold, low-salinity water characteristic of intense wintertime forcing. While possible feedback coupling of the atmosphere-ocean system remains to be determined, the direct correspondence of northwest Atlantic deep convection and cold-air outbreaks from Canada is not in doubt. The increasing supply of fresh water leading to the strong decline in salinity has several possible origins: increased flow of water and ice from the Arctic, decreased flow of saline waters northward in the Gulf Stream/North Atlantic Current system from the subtropical Atlantic, and both the increased hydrologic cycle and ice melt associated with global warming. The resemblance to the freshening seen in coupled global climate models may be misleading, because of the inaccuracy of the AO/NAO modes in many of the models. The enhanced AO/NAO mode actually increased the density and intensity of this intermediate-depth branch of the THC during this period, despite the decline in salinity. Cooling of the subpolar ocean during a general period of global warming is a dynamic negative feedback,

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Abrupt Climate Change: Inevitable Surprises FIGURE 2.10 Variation of salinity (A) and potential temperature (B) with time, at three depths in the Labrador Sea. There is extensive cooling and salinity decline beginning abruptly in 1972, lasting for the last quarter of the 20th Century. The “LSW” (Labrador Sea Water) responds to winter weather locally, while the deeper “NEADW” (Northeast Atlantic Deep Water) and “DSOW” (Denmark Strait Overflow Water) cascade over the sills east of Greenland, and feed the deeper layers of the world ocean. (Courtesy of I. Yashayaev, Bedford Institute of Oceanography.)

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Abrupt Climate Change: Inevitable Surprises which could delay by several decades the predicted slowing of the THC (e.g., Delworth and Mann, 2000). Numerical simulations suggest that this enhanced production of intermediate-level water (which forms the upper North Atlantic Deep Water) is enough to energize, strongly and quickly, the entire THC of the Atlantic, and its closely correlated meridional heat transport (Häkkinen, 1999; Cheng, 2000). Model simulations allow us to make connections between observable and unobservable quantities, for example the sea-surface elevation field (seen by satellite altimeter), meridional heat transport and meridional volume flux (Häkkinen, submitted). These model simulations emphasize the fast-track response of the intermediate-depth THC during abrupt swings of climate, which may be quite different from changes in the deeper branch of the THC driven by overflows from farther north. These changes during the instrumental era are in themselves large perturbations of the climate system. Still larger changes have been recorded in paleoclimate data, suggesting that complete shut-down of Labrador Sea deep convection, and its associated contribution to the THC, is possible. Hillaire-Marcel et al. (2001) inferred long periods without Labrador Sea convection during the previous interglacial period. Under a more heavily perturbed, warmer climate, we could see this state recur; learning to recognize its possible onset is a major goal of current research. North Atlantic Surface Salinity Events Important events embedded in the longer-term decadal variability of the Atlantic are the “great salinity anomalies,” which seem to involve out-pourings of low-salinity surface water and ice from the Arctic. One of the strongest developed during the late 1960s. The salinity of the upper subpolar Atlantic decreased during a series of mild winters at the same time that intense northerly winds occurred over the Fram Strait. It has been inferred that a great increase in Arctic ice and low-salinity water was driven into the Atlantic, although the influence is controversial. The combined effects of mild winters and buoyant surface layer all but halted deep convection in the Labrador and Irminger Seas. Low-salinity water was tracked as it progressed cyclonically around the high-latitude Atlantic (Dickson et al., 1988) and was seen moving into the Norwegian Sea in 1979; its influence was seen as far south as the Azores. “Lesser” salinity anomalies also appear in the instrumental record (Belkin et al., 1998). Finally, the long overall decrease in the salinity of the upper subpolar Atlantic since 1972 can similarly be at-

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Abrupt Climate Change: Inevitable Surprises tributed to the increasingly positive NAO events whose southerly winds near Europe tend to drive warm, saline Atlantic water into the Arctic (and, by inference, low salinity water back to the Atlantic) (Dickson et al., 2000). Those events provide models for the more intense salinity anomalies of the paleo-record, and perhaps of the near future. Changes in Arctic river runoff and its mixing into the Arctic Ocean may prove to be important (Dickson, 1999; Macdonald et al., 1999; Guay et al., 2001; Ekwurzel et al., 2001). Arctic Change in the 1990s The strong interaction between the Arctic and Atlantic Oceans was discussed above. During the recent period of widely varying climate in the subpolar Atlantic, the Arctic has also experienced great change. Exchange between Arctic and Atlantic is a new focus within the larger climate system. The increasing incidence of the positive phase of the AO/NAO appears to have driven warmer, more-saline Atlantic water into the Arctic and pushed back the boundary with fresher, colder waters of Pacific origin (Carmack et al., 1995, Morison et al., 1998). Warming of the polar Atlantic layer has been so striking as to be invoked in possible future melting of the sea ice cover. Attendant thinning of the sea-ice, by about 40 percent, has occurred in the central Arctic, based on comparison of data from 1993 to 1997 with those from 1958 to 1976 (Rothrock et al., 1999), but once again dynamics and greenhouse warming might be interacting or obscuring one another. Arctic ice-cover history is complex, yet full of strong climate signals (e.g., McLaren et al., 1990; Chapman and Walsh, 1993; Yueh and Kwok, 1998; Kwok et al., 1999). The relationship with AO/NAO wind forcing and temperature variability is potentially strong (Rigor et al., submitted). With positive polarity of the AO/NAO, weakening occurs of the typically high atmospheric pressure over the Beaufort Sea, together with intrusion of cyclonic flow related to the Icelandic low pressure center farther north into the Arctic. Moisture transport into the Arctic by this circulation has increased with the AO/NAO index (Dickson et al., 2000). Antarctic and Southern Ocean Climate Change Attention has focused on changes in the Arctic, but concern remains that this amounts to looking under the street light, missing other high-latitude sites that are less well observed. Southern Ocean overturning has two

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Abrupt Climate Change: Inevitable Surprises modes: through open ocean convection in polynyas (areas of open water that are surrounded by sea ice), in geographically suitable semi-enclosed seas (Ross and Weddell Seas), along the shelf of Australia, and driven by plumes of dense water descending from the shallow Antarctic shelf and slope, and interacting strongly with the overlying permanent ice. In contrast with the North Atlantic, the overturning that feeds the deepest waters is fed by cool mid-depth waters that first upwell near the Antarctic Circumpolar Current. Thus, the total heat transport associated with formation of dense waters in the Antarctic is not as dominant a part of the global heat budget as is the heat transport associated with the North Atlantic THC. However, variations in Antarctic overturning, and possibly surface warming, can affect freshwater budgets, as evidenced in large-scale freshening in the lower latitude oceans of the Southern Hemisphere (Wong et al., 1999; Johnson and Orsi, 1997). Both intermediate-depth and deeper branches of the THC form complex arteries of sinking and recirculating in the Southern Ocean (e.g., Orsi et al., 1999); both are important to the time-constants of response of the global ocean to abrupt change. The possible alteration of northern and southern sinking in the global THC, known as the see-saw, arises from model simulations (Stocker et al., 1992; Crowley, 1992) and from comparison of climate anomalies in Greenland and Antarctic ice-core records (e.g., Blunier and Brook, 2001; see Plate 2). Instrumental data are scarce, but the inventory of CFC and other transient tracers can be interpreted to suggest a recent weakening of sinking around Antarctica, in comparison with the longer-term signal seen in steady-state tracers related to nutrients and dissolved oxygen (Broecker, 1999). A suggested link is proposed between this oceanic behavior and the Little Ice Age, one of the major climate events of the past 1,000 years in the northern hemisphere. Identifying global connections in the climate system is a particularly important goal of modern observations. Toggweiler and Samuels (1995) have found that, using global ocean models, the THC is sensitive to forcing by westerly winds in the Southern Ocean (particularly those near the relatively narrow Drake Passage, south of South America). In their simulations the THC volume transport, as far north as the subpolar Atlantic, shifted in response to changes in this distant wind forcing. In 1972, a large hole opened in the ice cover of the Weddell Sea. This polynya lasted until 1974, and its progress was tracked by the early Nimbus satellites. The polynya was supported by the breakdown of the usual stratification leading to open-ocean convection (Gordon and Comiso, 1988).

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Abrupt Climate Change: Inevitable Surprises Elimination of the relative buoyancy of low-salinity upper waters allowed deep heat to be brought to the surface (so that the polynya can be selfsustaining; Martinson et al., 1981). The event left its mark on the ocean with anomalous cold, low-salinity deep water, which found its way into the deep circulation, as witnessed in the Argentine Basin farther north. Global Ocean Heat Content The archive of hydrographic data from throughout the world ocean gives us an instrumental record of climate change that integrates over time and space: the ocean dominates (by a factor of about 103) the heat capacity of the ocean-atmosphere system, and its storage of heat anomalies also greatly exceeds the land surface. Levitus et al. (1999) (also see Levitus et al., 2001 and Barnett et al., 2001) described the integrated ocean heat content from 1950 to 2000 (Figure 2.11). In every ocean, an overall warming during the last 50 years is evident. But in a hiatus 1978–1988, all the ocean basins other than the South Atlantic cooled substantially. The temporary cooling in Pacific and Indian Oceans (both north and south) amounted to about half the net rise (2 × 1023J) over the 50 years. The abrupt cooling is mostly in the top 300 m of the water column. The anomalous air-sea heat flux during the event averaged about 1 watt m–2 which is several times as large as the half-century-long warming (0.31 watt m–2). The cooling event fails to resemble the well-known dip in atmospheric surface temperatures between roughly 1940 and 1970 in timing or duration. Heat is mercurial, and global budgets like this can be surprisingly responsive to localized forcing. Possible contributors to the downward lurches are the eruptions of the volcanoes Agung in 1963, El Chichon in 1982, and Pinatubo in 1991 (Levitus, 2001). Couplings Instrumental records show preferred modes of behavior of the earth’s climate system. Furthermore, those modes might be coupled not only in time (such as the unknown relation between the few-year ENSO and fewdecade PDO variability), but also in space. Although the principal modes of variability at high latitude (the AO/NAO, and related southern hemisphere annular mode) have some independence from ENSO, connections have been proposed. For example, Hoerling et al. (2001) suggested, comparing observations and coupled-model simulations, that the average tropi-

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Abrupt Climate Change: Inevitable Surprises FIGURE 2.11 Time series for the period 1948 to 1998 of ocean heat content (1022J) in the upper 300 m for the Atlantic, Indian, Pacific, and world oceans. Note that 1.5 × 1022J equals 1 W•year•m–2 (averaged over the entire surface of earth). Vertical lines through each yearly estimate represent ±1 standard error (SE) of the estimate of heat content. (SOURCE: Levitus et al., 1999.) cal warming since 1950 is a cause of enhanced AO/NAO activity in the North Atlantic. This constitutes one pathway from anthropogenic forcing to high latitudes, and another comes via the stratosphere. Intensification of the stratospheric wintertime polar vortex under greenhouse-gas forcing has been a major prediction of models (e.g., Shindell et al., 2001). The AO/NAO is a mode of variability that grows more symmetric but no less energetic as one moves upward from troposphere to stratosphere (Perlwitz and Graf, 1995). Baldwin and Dunkerton (1999) demonstrated downward phase propagation of intense stratospheric anomalies. The possible intensification of the AO/NAO beneath an enhanced polar vortex is being investigated with models and observations (Shindell et al., 2001). Yet, crucial

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Abrupt Climate Change: Inevitable Surprises internal linkages, particularly the decadal shifts in Rossby wave propagation upward from the troposphere, still need to be established. ENSO has great reach, and can influence climate at high southern latitudes. A potentially important climate mode, the Antarctic Circumpolar Wave, involves the coupled ocean and atmosphere near the polar front, in the Southern Ocean (White and Peterson, 1996). This wavenumber-2 dislocation of the polar front propagates eastward at about 0.08 m. sec-1, thus taking 8 to 10 years to circumnavigate that ocean. The mode appears to be strongly excited by tropical ENSO events. At a much shorter timescale, blocking patterns in the strong westerly circulation in the southeast Pacific appear to be forced by ocean warming and divergence in the western tropical Pacific (Renwick and Revell, 1999). Couplings of this kind are achieved by Rossby-wave propagation. The modal behavior of earth’s climate is one of the major research results of the instrumental era. This review emphasizes these modes as well as describing recent examples of such regional changes as the Dust Bowl and North Atlantic oceanic conditions that were abrupt and impacted humans and ecosystems. The connection stems from the possibility that climate can lock into one phase or another of a modal oscillation; for example, preferring the warm equatorial Pacific phase of ENSO. Possible interactions among the major climate-system modes (especially ENSO and the high-latitude annular modes) suggest that changes in one could be propagated globally. The major perturbations associated with greenhouse-induced climate change may affect the likelihood of such changes, and their geographic extent. SYNOPSIS OF OBSERVATIONS A great wealth of evidence related to abrupt climate change has been collected, although important data gaps remain. Interpretation of sedimentary records to learn about past climate changes is improving rapidly. The unequivocal evidence of repeated large, widespread, abrupt climate changes in the past is striking. Such events were especially prominent during the cooling into and warming out of the most recent global ice age. However, the events are not restricted to cold times as shown by the cooling about 8,200 years ago, which punctuated conditions that in many places were similar to or even slightly warmer than recently. Other events during the current Holocene warm time may not have been as globally dramatic as the

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Abrupt Climate Change: Inevitable Surprises ice-age changes, but regional effects on water availability may have had large effects on humans and ecosystems. Documentation of past abrupt climate changes is quite good in certain regions such as from Greenland ice cores and some of the pollen and other records summarized above. But no reliable global maps of climate anomalies are available for any of the abrupt changes of the past. Well-dated records with high time resolution are especially scarce in the Pacific, the tropics, and southern high latitudes, but no ideal record yet exists and additional useful information can be obtained almost everywhere. Past changes in the hydrological cycle are especially poorly characterized relative to its importance to humans and ecosystems. The unavoidable incompleteness of the sedimentary record (spatially, temporally, and in variables recorded) means that changes from before the deployment of widespread instrumentation will never be understood as well as more recent changes. Instruments have not yet observed any of the large, global abrupt climate changes (a statement that some day may require modification in hindsight with respect to human-induced changes). However, regional events that satisfy the definition of abrupt climate change have been observed instrumentally. These are important in several ways: among others, they highlight observations required to characterize events and provide early warning of event onset and termination; they point to gaps in the observational system; and, they show the important role of coupled modes in such changes (and, by inference, perhaps also in the larger changes further in the past). Instrumental study of abrupt changes has drawn on a broad-based range of observations collected for many purposes. However, few of these data sets are targeted on those parts of the climate system that are believed to have participated in past abrupt changes or that are likely to exhibit abrupt and persistent changes when thresholds in the climate system are crossed. Documentation remains sketchy of ocean circulation, especially in regions of deepwater formation, and interactions with sea-ice processes and with freshwater fluxes from the atmosphere and from land hydrology including glaciers and permafrost. Gaps also remain in documentation of land-surface processes linked to hydrology, and of modal behavior of the coupled climate system. Much work is required to place all of the available observations into a coherent framework, capable of assimilating new observational data and contributing to an understanding of abrupt climate change. A start on such a framework is described in the next section.

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Abrupt Climate Change: Inevitable Surprises Innovative instrumental methods have been developed that offer us hope of detecting abrupt climate change when it appears. These include both new in situ probes for atmosphere and ocean and remote sensing from satellites. New sensors (for chemical and biological fields as well as physical quantities) and new platforms (moored, drifting, gliding or profiling) are becoming available in large numbers. They provide efficient coverage globally and locally, at lower cost than classical observation methods. In reviewing the abrupt changes seen during the instrumental period, it is obvious how much more would now be understood if just a few well-placed modern instruments had been in place for an extended time. For abrupt climate change, special emphasis on the dynamics of freshwater in both atmosphere and ocean is needed. Evaporation, precipitation, river flow, surface moisture, cryosphere dynamics, and related upper ocean dynamics are all relatively poorly observed, yet they are as important to climate as are the better observed thermodynamic fields. Cloud and water vapor dynamics in the atmosphere are crucial to climate, yet their representation in coupled numerical models is crude. Models that are heavily involved in predicting or diagnosing abrupt climate change are known to be inaccurate in representing various high-latitude oceanic processes. Among them are deep convection, sinking and overflow dynamics, flow through narrow passages, over sills and in the descending branch of the oceanic overturning circulation, sea-ice dynamics, and flow and thermodynamics of shallow shelf regions at high latitude. Discrimination between sinking in the Labrador Sea and sinking farther north, with overflow at the Greenland-Scotland ridge system, is not well handled by such models. For these reasons, sustained observations of the high-latitude ocean and its communication with the Arctic are needed. Hydrographic and chemical-tracer observations must be repeated, quasi-regularly, across important passages and major “stable” water masses. Direct observations of ocean circulation intensity using both in situ and satellite altimetric observations are particularly valuable for inferring meridional overturning variability. Satellite scatterometer wind fields give us a component of global air/sea interaction with remarkable resolution. Passive radiometric observations give us a wealth of temperature, sea-surface structure and moisture data, and possibly new fields such as surface salinity. Synthetic aperture radar observations from satellites provide high resolution detail of ice fields, upper ocean flows, winds and terrestrial flooding and drainage-basin flows. Tropical ocean observations near the sea-surface currently provide cov-

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Abrupt Climate Change: Inevitable Surprises erage of the pace of ENSO activity, and its connections with higher latitude. Fixed arrays, which have revealed so much about the Pacific, could be extended to Indian and Atlantic oceans, at least in skeletal form. The ocean near the coasts is a particularly sensitive and biologically productive region. Considering the close proximity of so many humans, it is surprising how poor the instrumental record is. As natural variability is overlaid on human-induced changes of many kinds, there is a lack of baseline observations for comparison. Our definition of climate and its impacts is expanding to include ecosystems and human activity, and in this spirit there are many more observations to be made, particularly aiming at biological fields. Regular coastal observational programs are hampered by overlapping jurisdictions and variable commitment to basic scientific questions. River systems are intimately connected with estuaries, and human modification again compounds natural climate variability of streamflow, water quality, and drainage patterns. Support for sustained observations in both coastal regions and watersheds should be of high priority.